Transport in gapped bilayer graphene: the role of potential fluctuations

We employ a dual-gated geometry to control the band gap \Delta in bilayer graphene and study the temperature dependence of the resistance at the charge neutrality point, RNP(T), from 220 to 1.5 K. Above 5 K, RNP(T) is dominated by two thermally activated processes in different temperature regimes and exhibits exp(T3/T)^{1/3} below 5 K. We develop a simple model to account for the experimental observations, which highlights the crucial role of localized states produced by potential fluctuations. The high temperature conduction is attributed to thermal activation to the mobility edge. The activation energy approaches \Delta /2 at large band gap. At intermediate and low temperatures, the dominant conduction mechanisms are nearest neighbor hopping and variable-range hopping through localized states. Our systematic study provides a coherent understanding of transport in gapped bilayer graphene.Comment: to appear in Physical Review B: Rapid Com

Isospin breaking and f0(980)f_0(980)-a0(980)a_0(980) mixing in the η(1405)→π0f0(980)\eta(1405) \to \pi^{0} f_0(980) reaction

We make a theoretical study of the $\eta(1405) \to \pi^{0} f_0(980)$ and $\eta(1405) \to \pi^{0} a_0(980)$ reactions with an aim to determine the isospin violation and the mixing of the $f_0(980)$ and $a_0(980)$ resonances. We make use of the chiral unitary approach where these two resonances appear as composite states of two mesons, dynamically generated by the meson-meson interaction provided by chiral Lagrangians. We obtain a very narrow shape for the $f_0(980)$ production in agreement with a BES experiment. As to the amount of isospin violation, or $f_0(980)$ and $a_0(980)$ mixing, assuming constant vertices for the primary $\eta(1405)\rightarrow \pi^{0}K\bar{K}$ and $\eta(1405)\rightarrow \pi^{0}\pi^{0}\eta$ production, we find results which are much smaller than found in the recent experimental BES paper, but consistent with results found in two other related BES experiments. We have tried to understand this anomaly by assuming an I=1 mixture in the $\eta(1405)$ wave function, but this leads to a much bigger width of the $f_0(980)$ mass distribution than observed experimentally. The problem is solved by using the primary production driven by $\eta' \to K^* \bar K$ followed by $K^* \to K \pi$, which induces an extra singularity in the loop functions needed to produce the $f_0(980)$ and $a_0(980)$ resonances. Improving upon earlier work along the same lines, and using the chiral unitary approach, we can now predict absolute values for the ratio $\Gamma(\pi^0, \pi^+ \pi^-)/\Gamma(\pi^0, \pi^0 \eta)$ which are in fair agreement with experiment. We also show that the same results hold if we had the $\eta(1475)$ resonance or a mixture of these two states, as seems to be the case in the BES experiment

Graphene nanoring as a tunable source of polarized electrons

We propose a novel spin filter based on a graphene nanoring fabricated above a ferromagnetic strip. The exchange interaction between the magnetic moments of the ions in the ferromagnet and the electron spin splits the electronic states, and gives rise to spin polarization of the conductance and the total electric current. We demonstrate that both the current and its polarization can be controlled by a side-gate voltage. This opens the possibility to use the proposed device as a tunable source of polarized electrons.Comment: 12 pages, 7 figures, accepted in Nanotechnolog

Power-laws in recurrence networks from dynamical systems

Recurrence networks are a novel tool of nonlinear time series analysis allowing the characterisation of higher-order geometric properties of complex dynamical systems based on recurrences in phase space, which are a fundamental concept in classical mechanics. In this Letter, we demonstrate that recurrence networks obtained from various deterministic model systems as well as experimental data naturally display power-law degree distributions with scaling exponents $\gamma$ that can be derived exclusively from the systems' invariant densities. For one-dimensional maps, we show analytically that $\gamma$ is not related to the fractal dimension. For continuous systems, we find two distinct types of behaviour: power-laws with an exponent $\gamma$ depending on a suitable notion of local dimension, and such with fixed $\gamma=1$.Comment: 6 pages, 7 figure

Ambient mixing ratios of nonmethane hydrocarbons (NMHCs) in two major urban centers of the Pearl River Delta (PRD) region: Guangzhou and Dongguan

The Pearl River Delta (PRD) region can be considered one of the most economically developed areas of mainland China. In September 2005, a total of 96 whole air samples were collected in Guangzhou and Dongguan, two important urban centers of the PRD region. Guangzhou is considered the economic center of Guangdong province, and Dongguan is a rapidly expanding industrial city. Here, we report mixing ratios of 50 nonmethane hydrocarbons (NMHCs) that were quantified in the ambient air of these PRD centers. The discussion focuses on understanding the main sources responsible for NMHC emissions, and evaluating the role of the identified sources towards ozone formation. Propane was the most abundant species in Guangzhou, with an average mixing ratio of 6.8 ppbv (±0.7 ppbv S.E.), compared to 2.5±0.2 ppbv in Dongguan. Toluene was the most abundant hydrocarbon in Dongguan (6.1±0.8 ppbv, compared to 5.9±0.7 ppbv in Guangzhou). Based on an analysis of the correlation between vehicular-emitted compounds and the measured NMHCs, together with the benzene-to-toluene (B/T) ratio, vehicular emission appears to be the dominant source of NMHCs measured in Guangzhou. By contrast, selected species (including toluene) in many of the Dongguan samples were influenced by an additional source, most likely related to industrial activities. A specific B/T ratio (<0.20) is proposed here and used as indicator of samples strongly affected by industrial emissions. The ozone formation potential (OFP) is calculated, and the role of the different NMHCs associated with industrial and combustion sources is evaluated. © 2008 Elsevier Ltd. All rights reserved

Vector magnetic field sensing by single nitrogen vacancy center in diamond

In this Letter, we proposed and experimentally demonstrated a method to detect vector magnetic field with a single nitrogen vacancy (NV) center in diamond. The magnetic field in parallel with the axis of the NV center can be obtained by detecting the electron Zeeman shift, while the Larmor precession of an ancillary nuclear spin close to the NV center can be used to measure the field perpendicular to the axis. Experimentally, both the Zeeman shift and Larmor precession can be measured through the fluorescence from the NV center. By applying additional calibrated magnetic fields, complete information of the vector magnetic field can be achieved with such a method. This vector magnetic field detection method is insensitive to temperature fluctuation and it can be applied to nanoscale magnetic measurement.Comment: 5 pages, 5 figure